Allow bumping GH actions to rolling releases

[evgeni]

Currently dependabot will bump GitHub actions to whatever the latest vX.Y.Z tag is. However, actions also usually have a vX tag that points to the latest X.Y.Z (and gets updated). I'd like to be able to select the bumping behavior and prefer vX tags.

Example PR that was opened with full version and then manually changed to v2: theforeman/releasetool#19

[evgeni]

cc @ekohl

[nickserv]

I've noticed this is also an issue when the same major version tag has an update. For example, v2 should not be bumped to v2.0.1.

I think we should respect the existing tag precision to stay consistent with #4953. GitHub Actions docs tend to use major version tags anyway, so this should still simplify Dependabot usage for those users.